When using neutron polling mode, booting a VM will timeout because
nova-compute cannot receive network-vif-plugged event. This is
because we set vif['id'](i.e. neutron port uuid) to two different
OVS ports. One is Xenserver VIF, the other is tap device qvo-XXXX
which neutron ovs agent will monitor and deal with.
My previous patch https://review.openstack.org/#/c/463246/ has
deprecated the setting of 'nicira-iface-id' to XenServer. This
patch is to remove the setting of 'nicira-iface-id'.
Closes-Bug: #1732624
Change-Id: I221e89686c8e9041651f5affc4e3542d75ff55bc